Solvent-free two-component (2K) polyurethane or polyurea coating compositions, by contrast with solventborne paints, can be applied in coating thicknesses virtually as high as desired. They are used nowadays essentially in the construction sector for production of thick-build coatings, for example for coating of mechanically or chemically highly stressed areas, for example industrial floors, traffic areas, parking decks, for balcony sealing, or else in corrosion protection.
For outdoor applications, there is particular interest here in highly elastic crack-bridging coatings which have high stability to weathering stress, especially with respect to UV radiation.
Solvent-free 2K coatings necessarily require liquid co-reactants of low viscosity. While a number of suitable polyols and polyamines having sufficiently low viscosity, for example polyether polyols, lightly branched polyester polyols, polyether amines or polyaspartic esters are available, crosslinker components used for production of elastic coatings are preferably oligomeric polyisocyanates based on linear aliphatic diisocyanates, especially polyisocyanates of hexamethylene diisocyanate (1,6-diisocyanatohexane, (HDI)).
HDI polyisocyanates, such as the derivatives of HDI having uretdione, isocyanurate, allophanate, biuret, iminooxadiazinedione and/or oxadiazinetrione structure, which are known as paint polyisocyanates, even in solvent-free form, feature low viscosities and, as is well known, afford flexible paint films of high weathering resistance.
However, the maximum elasticity achievable with these oligomeric paint polyisocyanates is insufficient for production of crack-bridging coatings. There is reliance here on the use of linear or lightly branched isocyanate-functional prepolymers (NCO prepolymers) as crosslinker components.
Liquid low-monomer HDI prepolymers are known in principle. WO 2004/033517 describes, for example, NCO prepolymers prepared by reaction of polyisocyanates or diisocyanates, for example HDI, with polyether polyols, as co-reactants for polyaspartic esters for production of elastic coatings. Polyether prepolymers generally have very low viscosities, but are only of limited suitability because of their poor weathering resistance—a consequence of oxidative polyether chain degradation—for outdoor applications (U. Meier-Westhues, Polyurethane—Lacke, Kleb- and Dichtstoffe [Polyurethanes—Paints, Adhesives and Sealants], Hanover: Vincentz Network 2007, p. 50).
NCO prepolymers based on aromatics-free polyester polyols, by contrast, lead to coatings having good weathering stability and high gloss. This applies both to prepolymers based on polyester polyols that have been prepared by polycondensation of aliphatic di- and polycarboxylic acids or the anhydrides thereof with excess amounts of polyfunctional alcohols and to polycaprolactone polyols that have been obtained by ring-opening condensation of ε-caprolactone.
However, the polyester prepolymers of HDI known to date also have disadvantages. While those that have been prepared using condensation products of linear dicarboxylic acids have considerably higher viscosities than comparable polyether prepolymers or are even solid at room temperature, polycaprolactone polyester prepolymers of HDI exhibit comparatively low viscosities but also a strong tendency to crystallize, which leads to a rapid increase in haze and solidification especially under cool ambient conditions.
Isocyanate-functional HDI polyester prepolymers which fulfill all practical requirements, especially a sufficiently low viscosity for manual processing coupled with a simultaneously low tendency to crystallize, and which lead to elastic coatings of high mechanical durability are unknown to date.